Upgrade question for newbie

A few months ago I acquired my first system:

1. Magnepan 1.6 speakers
2. Emotiva UPA-2 amp (185 watts into 4 ohms)
3. Marantz DV6001 universal player running directly to amp

I am greatly enjoying my system, and I plan to upgrade it as funds allow. My room (a media room/game room) is approximately 23 x 25. I listen to music much more than I watch movies.

My question: If I were to spend about $1,000 or so for the first upgrade, which of the following options do you think would give me more bang for my buck:

1. A better/more powerful amp (although I am very pleased with the Emotiva for the price and it seems to drive the Magnepans better than a couple of different NAD models that I heard at the dealer)
2. A DAC (e.g., the new Peachtree iDecco or V-DAC)
3. A subwoofer (e.g., a used REL or Martin Logan)
4. A preamp
5. Something else

Thanks!